The present invention relates to a system for compensating for a drop in idling speed of an engine taking place when the engine starts to be subjected to load after a manual valve has been shifted from a neutral range position to one of a plurality of drive range positions.
An automatic transmission is known which stays in a neutral state when a driver places a manual valve at a neutral range ("N" range) position or a parking range ("P" range) position. When the driver shifts the manual valve at one of drive range positions including an automatic drive range ("D" range), at least one manual drive range, and a reverse drive range, the automatic transmission shifts to a torque transmission state where at least one of friction elements is engaged to establish a power delivery path.
With the vehicle at a standstill, if the driver shifts the manual valve from the neutral range position to the drive range position, a start-up friction element adapted to be engaged to establish a power delivery path for the first gear position or speed or a start-up friction element adapted to be engaged to establish a power delivery path for the reverse gear position is hydraulically activated by a servo activating hydraulic fluid pressure (line pressure) delivered thereto from the manual valve. Thus, the vehicle becomes ready for start running.
Considering the variation of the idle speed during this select operation by the driver, since the vehicle is at a standstill, there occurs a creep in the torque converter, inducing a load which is applied to the engine, causing a drop in the idling speed of the engine, leading to increased tendency of the engine to stall.
In order to compensate for this undesired drop in idling speed, it has been proposed to increase the amount of intake air in timed with the occurrence of the driver's shifting the manual valve from the neutral range position to the drive range position based on output of an inhibitor switch which is provided to detect which range position the manual valve is placed at.
This proposal is insufficient in that there is a likelihood that the engine speed is increased before the corresponding start-up friction element is hydraulically activated, inducing substantial shock upon actual engagement of the start-up friction element. This is derived from a delay between the instance when the manual valve is shifted to the drive range position and the instance when the servo activating hydraulic pressure is applied to activate the start-up friction elemenmt. This delay is not constant and differs with aging of the component parts of the automatic transmission, temperature of the hydraulic fluid, idling speed, and etc., making it very difficult to increase the engine speed in timed with the actual engagement of the start-up friction element in all circumstances.
An object of the present invention is to improve the above-mentioned compensating system such that the engine torque is increased in timed with the actual engagement of the start-up friction element after the manual valve has been shifted from the neutral range position to the drive range position in all circumstances.